South Jersey Skies: A drive on the Moon

Would you like to drive a vehicle on the Moon? Quite a number of people have done so already. But the guys who drove the first one never set foot on the Moon!

You’ve probably seen a movie of the Apollo astronauts zipping across the lunar surface in a NASA lunar rover (if you haven’t, look at wn.com/Apollo_16_Lunar_Rover_video_clip). But five Russians drove their lunar rover on the Moon more than eight months before the first American rover even arrived.

View full sizeGoddard Space Flight Center, NASALunakhod 1 rover vehicle. The retroreflector array can be seen on the end of the arm to the left.

It was called Lunokhod 1, and it landed in Mare Imbrium (the “Sea of Rains”) 40 years ago next Wednesday. There were no humans on board the Luna 17 rocket that carried it to the Moon. L1 was the first roving remote-controlled robot ever to move about on another world.

You probably never heard of Lunokhod 1, or its later brother, Lunokhod 2. These hardy little explorers were forgotten in the blaze of publicity for the American Apollo missions then underway. But L1 carried lots of scientific equipment: an X-ray spectrometer, a cosmic-ray detector, four cameras, and a penetrometer (a “ground stomper,” as one developer named it) that allowed it to test the rigidity and fragility of the lunar soil.

The one-ton Lunokhod was powered by the Sun. Controlled in real time by a team of five engineers at the Deep Space Center near Moscow, its eight wheels transported it across nearly seven miles of gray dust and small craters during its mission.

Designed to function for 90 days, the rover continued to send back valuable data for more than 11 months. We collected over 20,000 images, including high-resolution panoramas of the surrounding countryside in several spots.

And believe it or not, we’re still getting back information from the rover. Both of the Lunokhods carried a small array of mirrors called a retroreflector. Such a device is designed to reflect light beams back exactly along the direction from which they came.

Using a large telescope, astronomers aim a very bright beam of laser light at a lunar retroreflector. Only a few photons out of the gazillions in the beam bounce off the mirrors and make it back to Earth successfully. By precisely measuring how long it takes them to complete their journey — about a second and a half — we obtain a remarkably exact figure for the distance to the Moon at that instant. We can now measure how far away the Moon is to within a millimeter.

By making repeated measurements, we’ve confirmed that the force of gravity is basically stable, not changing over intervals of at least a few years. We have tested the Theory of Relativity and advanced our understanding of gravity in general. We also know now that the Moon is moving away from the Earth about a centimeter each year.

Until this year, we had only four retroreflectors we could use: three that were left by Apollo missions and one that was attached to Lunokhod 2. We couldn’t use the fifth one on Lunokhod 1 because we didn’t know where the rover even was. Its location on the Moon was not pinpointed well enough to aim our lasers.

But early this year, scientists were able to finally find L1 on images taken by the Lunar Reconnaissance Orbiter satellite. You can see what they found at tinyurl.com/2bhbmf7.

Once they had an accurate position, they could shoot a laser beam precisely at the spot, hoping that they might see a few photons coming back. Surprisingly, the mirrors on L1 worked about four times better than those of L2, which they’d been using successfully for years. The signal was quite strong: about 200 photons came back in each reflected pulse. This was enough to detect even in daylight. Finding Lunokhod 1 has increased our retroreflector information flow by 25 percent. Not bad for a vehicle that’s been sitting unappreciated on the Moon for 40 years!

------

Today at the Planetarium. On the second Sunday of most months the planetarium has been hosting a variety of family shows, intended for families with young children. Usually we offer one of our regular school programs for the purpose. But this afternoon we’re trying something different.

“Peter and the Wolf” is a familiar story, and it’s also a charming piece of classical music by Sergei Prokofiev, in which each of the major characters is represented by a particular instrument, and a narrator tells the story, accompanied by a symphony orchestra.

In this afternoon’s version, we’ll add lasers. Each character is drawn with colorful animated outlines produced by the planetarium’s laser projector. The cat and the duck are comical, and the wolf is appropriately menacing; and, of course, Peter is properly heroic. Orchestral interludes are accompanied by dancing abstract laser patterns. The English musician Sting narrates the story.

“Laser Peter and the Wolf” is enjoyable by children of all ages — and adults as well. The show begins at 3 p.m. More information can be found at the planetarium website, www.rowan.edu/planetarium/.